International achievement comparison studies assess students on core subjects such as Reading, Mathematics and Science. Students who do not speak the test language at home can be expected to be disadvantaged because of language proficiency. The test language effect has not been given sufficient attention. The present study investigated probable test language effect by using as data the country means reported in Reading, the PISA 2009 Reading. There was a wide range of proportions of non-speaker of test language among the participating countries. The average proportion of test language speakers is 80% with a wide standard deviation of 22%. The Reading mean for test language speakers is 39.2 points greater the that for non-speakers and the effect size is Cohen’s d = .69. An adjusted Reading means to off-set test language effect was suggested. Careful scrutiny of the differences between original and adjusted means indicates that the test language effect is not simply linear. Effectiveness in second-language teaching may account for this complexity. Further research is indicated.